diff --git a/lib/Slic3r/Config.pm b/lib/Slic3r/Config.pm
index 35aaf1dc4..34738e5b5 100644
--- a/lib/Slic3r/Config.pm
+++ b/lib/Slic3r/Config.pm
@@ -169,7 +169,7 @@ sub validate {
     die "Invalid value for --first-layer-height\n"
         if $self->get_value('first_layer_height') <= 0;
     die "Adaptive slicing requires a non-relative first layer height."
-        if $self->get_value('adaptive_slicing') && $self->first_layer_height ~ /^(?:\d*(?:\.\d+)?)%$/;
+        if $self->get_value('adaptive_slicing') && $self->first_layer_height ~= /^(?:\d*(?:\.\d+)?)%$/;
     
     # --filament-diameter
     die "Invalid value for --filament-diameter\n"